ORGANIC LETTERS À 2013 Fluevirosines A C: A Biogenesis Inspired Vol. 15, No. 1 Example in the Discovery of New Bioactive 120–123 Scaffolds from Flueggea virosa Hua Zhang, Chuan-Rui Zhang, Kong-Kai Zhu, An-Hui Gao, Cheng Luo, Jia Li, and Jian-Min Yue* State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, 555 Zuchongzhi Road, Zhangjiang Hi-Tech Park, Shanghai 201203, People’s Republic of China [email protected] Received November 15, 2012 ABSTRACT Biogenesis inspired chemical investigation of a Chinese folk medicine, Flueggea virosa, returned three unprecedented C,C-linked trimeric Securinega alkaloids, fluevirosines AÀC(1À3). Their absolute structures were characterized on the basis of spectroscopic data and computational analysis. Compounds 2 and 3 showed inhibition against the splicing of XBP1 mRNA. The plant originated Securinega alkaloids are a group of rheumatoid arthritis, etc.,4 has proven to be a rich source of secondary metabolites isolated mainly from the species of Securinega alkaloids.5 Fluggeaine ether5a from F. virosa, the Securinega, Phyllanthus, Flueggea, Margaritaria,and the first dimer of this alkaloid family, is likely the conden- Breynia genera of the Euphorbiaceae family.1 Atypical sation product of two norsecurinine-type cometabolites structure of these alkaloids comprises an indolizidine via an ether bridge. Considering the R,β,γ,δ-unsaturated (securinine-type)/pyrrolizidine (norsecurinine-type) het- carbonyl moiety that is prone to attack from nucleophiles, erocycle and an R,β,γ,δ-conjugated lactone fragment to the report of fluggeaine ether is not surprising, while the form a highly rigid tetracyclic skeleton. Due to their exploration of more complex oligomers also appears fascinating structural features and significant biological prospective. However, despite the rapid development of properties, the Securinega alkaloids have attracted con- siderable interest from natural products, synthetic, and medicinal chemists, as well as pharmacologists in the past (4) Li, B.; Gilbert, M. G.; Fischer, G.; Meyer, C. A. In Flora of China; 1,2 Wu, Z. Y., Raven, P. H., Hong, D. Y., Eds.; Science Press: Beijing, Missouri half century, since securinine was initially discovered Botanical Garden Press: St. Louis, 2008; Vol. 11, p 178. from Securinega suffructicosa.3 (5) (a) Chen, M. J.; Hou, L. L. Zhiwu Xuebao 1985, 27, 625–629. (b) Luger, P.; Weber, M.; Dung, N. X.; Ky, P. T.; Long, P. K. Acta Flueggea virosa (Roxb. ex Willd.) Voigt, whose parts are Crystallogr., Sect. C 1995, 51, 127–129. (c) Dehmlow, E. V.; Guntenhoner,€ used in Chinese folk medicine for the treatment of eczema, M.;Ree,T.V.Phytochemistry 1999, 52, 1715–1716. (d) Gan, L. S.; Fan, C.Q.;Yang,S.P.;Wu,Y.;Lin,L.P.;Ding,J.;Yue,J.M.Org. Lett. 2006, 8, 2285–2288. (e) Gan, L. S.; Yue, J. M. Nat. Prod. Commun. 2006, 1, 819–823. (1) For reviews, see: (a) Snieckus, V. In The Alkaloids; Manske, R. H. F., (f)Wang,G.C.;Wang,Y.;Li,Q.;Liang,J.P.;Zhang,X.Q.;Yao,X.S.;Ye, Ed.; Academic Press: New York, 1973; Vol. 14,pp425À506. (b) Beutler, J. A.; W. C. Helv. Chim. Acta 2008, 91, 1124–1129. (g) Wang, G. C.; Liang, J. P.; Brubaker, A. N. Drug Future 1987, 12, 957–976. Wang,Y.;Li,Q.;Ye,W.C.Zhongguo Tianran Yaowu 2008, 6, 251–253. (h) (2) For reviews, see: (a) Zhang, W.; Li, J. Y.; Lan, P.; Sun, P. H.; Zhao,B.X.;Wang,Y.;Zhang,D.M.;Jiang,R.W.;Wang,G.C.;Shi, Wang, Y.; Ye, W. C.; Chen, W. M. J. Chin. Pharm. Sci. 2011, 20, 203– J. M.; Huang, X. J.; Chen, W. M.; Che, C. T.; Ye, W. C. Org. Lett. 2011, 13, 217. (b) Weinreb, S. M. Nat. Prod. Rep. 2009, 26, 758–775. 3888–3891. (i) Zhao, B. X.; Wang, Y.; Zhang, D. M.; Huang, X. J.; Bai, (3) Murev’eva, V. I.; Ban’kovskii, A. I. Dokl. Akad. Nauk SSSR L.L.;Yan,Y.;Chen,J.M.;Lu,T.B.;Wang,Y.T.;Zhang,Q.W.;Ye, 1956, 110, 998–1000. W. C. Org. Lett. 2012, 14, 3096–3099. 10.1021/ol303146a r 2012 American Chemical Society Published on Web 12/17/2012 separation and NMR techniques in the past two decades, and strong (1755 cmÀ1,CdO) absorption bands corre- the first CÀC linked dimeric flueggenines A (4) and B were sponding to olefinic group(s) and R,β-unsaturated only isolated and characterized from F. virosa in 2006 by γ-lactone(s).8 The NMR data for 1 (Table S1 in SI) showed our research group, with the former showing cytotoxicity high similarities to those for flueggenine A (4)5d and (À)- against the P-388 tumor cell line.5d norsecurinine (5)5a (Table S2 in SI), exhibiting resonances for three lactone carbonyls (δC 172.6, 172.1, and 171.9), three trisubstituted double bonds (δC 173.9, 110.0; 168.5, 107.4; 139.0, 132.2; δH 6.68, 5.68, and 5.66), one tetrasub- 3 stituted double bond (δC 164.9 and 122.1), and 25 sp carbons (15 methylenes, eight methines, and three oxyge- nated quaternary carbons at δC 91.9, 91.8, and 89.9). These data accounted for seven DBEs and suggested the presence of a 12-ring system in 1. The above observations indicated that 1 was likely a trimeric Securinega alkaloid consist- ing of one norsecurinine and two dihydronorsecurinine substructures. In the previous paper, we proposed a plausible biosyn- thetic pathway for flueggenines A (4) and B involving a key self-catalyzed step similar to that of the BaylisÀHillman reaction.5d Inspired by the discovery of the two dimers and the fact that they still possess a less hindered C-130 (as marked in 4, Scheme 1) active site, we assumed that a higher level of oligomers could also exist in a trace of amount in this herb. To challenge this hypothesis, we Figure 1. Key 2D NMR correlations for 1. recollected the plant material of F. virosa and performed a more careful and intensive investigation. Confirming our Further analysis of the 2D NMR data (1HÀ1HCOSY, assumption, the current project resulted in the separation HSQC, and HMBC; Figure 1 and Figures S3ÀS5 in SI) and identification of three unprecedented trimeric alka- enabled a full assignment of all signals and the construction loids, fluevirosines AÀC(1À3), together with their mono- of a trimeric architecture for 1 as shown. More specifically, meric precursors (À)-norsecurinine (5),5a viroallosecurinine the 1HÀ1H COSY data (Figure 1) revealed diagnostic (6),6 and virosecurinine (7),5a as well as the dimeric structural fragments that well matched those in 55a and intermediate flueggenine A (4).5d Absolute structures dihydronorsecurinine,9 i.e., (a) H-2 to H -5 and H -8 to were assigned to 1À3 based on detailed examinations of 2 2 H-15 via H-7 for substructure A; (b) H-20 to H -50 and spectroscopic data, comparisons with the authentic co- 2 H -80 to H-140 via H-70 and H-150 for substructure B; metabolites 4À7 (for structures, see Chart S1 in Supporting 2 (c) H-200 to H -500 and H -800 to H-1400 via H-700 and H-1500 Information (SI)), computational analysis, and biogenetic 2 2 for substructure C. In addition, the three structural units as considerations. We herein describe the extraction, isola- one norsecurinine and two dihydronorsecurinines were tion, structural elucidation, and bioactivities of these also confirmed by HMBC data (Figure 1) as depicted. In intriguing alkaloids. particular, the critical HMBC correlations from H-150 to High resolution EIMS analysis of fluevirosine A (1)7 C-13, C-14, and C-15, and from H-1500 to C-110,C-120,and revealed a molecular ion at m/z 609.2838 consistent with a C-130, unambiguously established the connections of molecular formula of C H N O (calcd 609.2839) incor- 36 39 3 6 C-150/C-14 and C-1500/C-120, thus defining the planar porating 19 double bond equivalents (DBEs). The IR À structure of 1 as drawn. spectrum displayed medium (1643 and 1624 cm 1,CdC) The relative configuration of 1 was characterized by (6) Tatematsu, H.; Mori, M.; Yang, T. H.; Chang, J. J.; Lee, T. T. Y.; interpretation of ROESY data (Figure S6 in SI) and Lee, K. H. J. Pharm. Sci. 1991, 80, 325–327. comparisons with model compounds 45d and flueggine B.5h 23 (7) Pale yellow solid; [R]D þ13.5 (c 0.855, CHCl3); UV (MeOH) λmax (log ε) 259 (4.43), 212 (4.78) nm; CD (MeOH) λ (Δε) 266 (À11.8) € nm; IR (KBr) νmax 2962, 2872, 1755, 1643, 1624, 1460, 1223, 1118, 1080, (8) Pretsch, E.; Buhlmann, P.; Badertscher, M. In Structure Deter- 919 cmÀ1; 1H and 13C NMR data, see Table S1 in SI; EIMS (70 eV) m/z mination of Organic Compounds, 4th ed.; Springer-Verlag: Berlin and 609 [M]þ (14), 540 (78), 500 (8), 471 (6), 350 (14), 204 (6), 190 (10), Heidelberg, 2009; pp 273À275 and 317. þ 96 (29), 70 (100); HR-EIMS m/z 609.2838 [M] (calcd for C36H39N3O6, (9) Han, G.; LaPorte, M. G.; Folmer, J. J.; Werner, K. M.; Weinreb, 609.2839). S. M. J. Org. Chem. 2000, 65, 6293–6306. Org. Lett., Vol. 15, No. 1, 2013 121 00 of 2 was substituted. The remarkably shielded C-8 (ΔδC À4.7 ppm) in 2 compared to C-8 in dihydroviroallosecur- inine further confirmed the above assignment requiring 00 00 the 15 -substituent to be coplanar with CH2-8 .